The present invention relates to a method for the preparation of conjugated polymers. In particular, the present invention relates to a method for the topochemical polymerization of two molecules using the supramolecular characteristics of the molecules to design molecular structures from conjugated monomers having greater than two unsaturated bonds.
Supramolecular chemistry is one of the most rapidly developing fields of science due to the importance of intermolecular interactions, structure, and cooperativity in molecular science and many related technological fields, including biology, pharmacology and materials science. Supramolecular chemistry is often referred to as the chemistry of the non-covalent bond. The forces involved in non-covalent bonds are relatively weak, non-directional and occur over the entire van der Waals surface of a molecule. The preparation of a supramolecular structure requires controlling these weak, non-directional forces in three dimensions. This is a formidable problem beyond current abilities, since it is generally not possible to even predict the solid state structure of a given molecule.
The physical and chemical properties of a molecular solid depend directly upon the relative orientations and spacings of the constituent molecules. In order to produce synthetic forms of a solid, it is necessary to duplicate the relative orientations and spacings. The duplication of the relative orientations and spacings of complex crystal structures poses numerous problems. Attempts have been made to approach the problem by combining methods of traditional molecular synthesis with the techniques of supramolecular synthesis. However, these attempts have been mostly unsuccessful because a method has not been found to control the molecular orientations and spacings in a crystal to produce a synthetic form of a solid.
Substituted ureas and vinylogous ureas have been used to prepare a variety of layered organic crystals. The urea functionality consistently forms one-dimensional hydrogen-bonded xcex1-networks via hydrogen bonds between the carbonyl of one urea and the anti-hydrogen atoms of the nitrogen atoms of a neighboring urea. The xcex1- and xcex2-networks are supramolecular structural features possessing translational symmetry in one and two directions, respectively. These networks, together with discrete assemblies and xcex3-networks, represent the four fundamental supramolecular structures.
Supramolecular structures are discussed in detail in an article titled, An Approach to the Design of Molecular Solids. The Ureylene Dicarboxylic Acids by X. Zhao, Y. L. Chang, F. W. Fowler and J. W. Lauher, Journal of the American Chemical Society, 1990, 112, 6627, which is incorporated herein by reference in its entirety. If substituents, such as carboxylic acid functionalities, are added to the urea, the one-dimensional xcex1-networks can be brought together to form a two-dimensional xcex2-network. By choosing the appropriate molecule, the spacings and symmetry of the hydrogen-bonded xcex2-networks that define these layers can be controlled.
The families of polymers derived from acetylene and its oligomers have been widely explored for the development of advanced materials. Polyacetylenes can be prepared by the polymerization of alkynes in solution to form a series of acetylenes, which includes polyacetylene, polydiacetylene and polytriacetylene. FIGS. 3a, 3b and 3c show the structures of polyacetylene, polydiacetylene and polytriacetylene, respectively. Until now, only the first and second members of this series (polyacetylene and polydiacetylene) have been prepared by direct polymerization of the monomer. The polydiacetylenes can only be prepared by the topochemical polymerization of diacetylenes in a condensed phase. The third member of this series, polytriacetylene, has not been successfully polymerized to produce a polytriacetylene.
Polydiacetylenes have been formed by a unique synthesis via topochemical polymerization of diacetylenes in a condensed matter state. It has been found that polymerization occurs when the monomer molecules are properly aligned at a critical repeat distance (d) of approximately 5.0 xc3x85 and with an orientation angle (xcfx86) of about 45xc2x0 relative to the translation axis. For such a 1-4 polymerization to take place, the monomer molecules must meet stringent structural requirements, so that the C1 and C4 carbons of neighboring monomers are in close intermolecular contact. However, most diacetylenes do not crystallize properly and most do not polymerize at all in the solid state because there is little that can be done to influence the crystal structure of a single molecule.
The topochemical requirements for the polymerization of diacetylenes are disclosed by V. Enkelmann in Structural Aspects of the Topochemical Polymerization of Diacetylenes, Advanced Polymer Science, 1984, 63, 91-136. This reference is incorporated herein in its entirety. Enkelmann discloses an intermolecular repeat distance, d, of approximately 4.9 xc3x85 and a tilt angle, xcfx86, of approximately 44xc2x0 from the axis brings the 1-4 carbons of adjacent diacetylenes into near van der Waals contact. A diacetylene that meets these structural requirements can be expected to undergo topochemical polymerization upon radiation or heat exposure. The difficulty is that most diacetylenes do not meet these structural requirements upon crystallization. Accordingly, only a few diacetylenes polymerize to produce polydiacetylenes
Polytriacetylenes are a new class of conjugated polymer networks that are derived from polyacetylene and polydiacetylene. Oligomers of this polymer have been formed, not by the direct polymerization of a triacetylene, but only by indirect means. The polytriacetylene oligomers have attractive electrical and optical properties and they appear to be more stable than either the polyacetylenes or the polydiacetylenes. Stability is an important property since the instability of polyacetylenes is one of the primary reasons that this class of conjugated polymers has resisted commercial development.
The obstacles that must be overcome for triacetylene polymerizations are even more challenging than those encountered in diacetylene polymerizations. Polytriacetylene oligomers have been prepared by indirect means but not by polymerization of a triacetylene. Significantly, polytriacetylene oligomers appear to be more stable than either the polyacetylenes or polydiacetylenes. Previous attempts to polymerize triacetylenes have not resulted in the reporting of a successful example of this transformation.
Attempts to form conjugated polymers, such as polytriacetylene, have been mostly unsuccessful because most complex polymers, such as triacetylene, do not crystallize properly and most do not polymerize at all in the solid state. The present invention has overcome these problems by developing a strategy that takes advantage of supramolecular characteristics of different molecules to design and build co-crystal structures, which can be polymerized to form conjugated polymers.
The present invention is a method for the preparation of a conjugated polymer from a conjugated monomer that is not directly polymerizable. In one embodiment, the conjugated monomer has greater than two unsaturated bonds. The method includes complexing a stable host molecule and a guest conjugated monomer having greater than two unsaturated bonds to form a co-crystal. The complexing fixes the conjugated monomer such that when a plurality of the co-crystals are polymerized, reactive atoms of adjacent monomers are brought into near van der Waals contact to each other. After the co-crystals are formed, they are polymerized to form a conjugated polymer. In one embodiment of the present invention, the method includes determining a first intermolecular repeat distance and a first tilt angle relative to the translation axis for the guest conjugated monomer prior to complexing with the host molecule. In another embodiment, the host molecule has a second intermolecular repeat distance and a second tilt angle relative to the translation axis, and the guest conjugated molecule is selected so that the first and the second repeat distances and the first and the second tilt angles are substantially equal.
The preferred method of polymerizing the co-crystals is topochemical polymerization, which is preferably carried out using gamma radiation. Preferably, at least fifty percent of the co-crystals are converted to a conjugated polymer when the co-crystals are polymerized and most preferably at least seventy percent of the co-crystals are converted.
In a preferred embodiment of the present invention, polytriacetylene is prepared by complexing a stable host molecule and guest triacetylene molecule to form a co-crystal. The complexing fixes the triacetylene molecule so that when a plurality of the co-crystals are polymerized, reactive atoms of adjacent triacetylene molecules are brought into near van der Waals contact to each other, preferably within about 3.5 xc3x85. The co-crystals are then polymerized to form polytriacetylene. It has been found that the preferred reactive atoms of the adjacent triacetylene molecules, which are brought into near van der Waals contact, are the C1 and C6 atoms. The triacetylene molecules have a first intermolecular repeat distance of about 7.5 xc3x85 and a first tilt angle relative to the translation axis of about 27xc2x0. In a preferred embodiment, a host molecule is selected which has a second intermolecular repeat distance, preferably from about 7.1 xc3x85 to about 7.6 xc3x85, and a second tilt angle relative to the translation axis, preferably from about 23xc2x0 to about 32xc2x0, so that the first and the second repeat distances and the first and the second tilt angles are substantially equal. The preferred host molecules are isocytocine, aminopyridone, vinylogous amide and diaminobenzoquinone.
The present invention provides a method of producing conjugated polymers, such as polytriacetylenes, which possess valuable optical and electrical properties useful for the development of advanced materials.